fix: typeError in canonicalize() when user_id/device_id are non-strings

[vaibhav-jain-exp]

Summary

Fixes the following issues:

• TypeError: _a.trim is not a function during exposure event #69
• prevents trim is not a function exception #70

Checklist

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[macroscopeapp]

Prevent TypeError and undefined literals by using util.safeStringTrim in assignment.Assignment.canonicalize and exposure.Exposure.canonicalize to sanitize user_id, device_id, key, and variant.key

Introduce util.safeStringTrim and update canonicalization in assignment and exposure to trim and coerce non-string identifiers, returning empty strings for null/undefined.

📍Where to Start

Start with util.safeStringTrim in string.ts, then review its use in assignment.Assignment.canonicalize in assignment.ts and exposure.Exposure.canonicalize in exposure.ts.

Changes since #71 opened

• Added test coverage for Assignment.canonicalize, Exposure.canonicalize, and safeStringTrim utility function [03072dd]

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Macroscope summarized 1b553b9.

[kyeh-amp]

Looks good, but could you add a unit test to check the original issue?